新疆蕴都卡拉A型碱性花岗岩岩石地球化学特征及其构造意义

Geochemistry and its tectonic implication for A-type alkali granite in Yundukala area,Xinjiang

新疆准噶尔东北缘蕴都卡拉一带碱性花岗岩具有高硅(w(SiO_2)=72.40%-74.17%)、富碱(w(Na_2O+K_2O)=8.41%-10.40%)、低钛(w(TiO_2)=0.20%-0.40%)、贫钙(w(CaO)=0.38%-0.70%)的特征,岩石显著富集高场强元素Zr、Th、Y等,轻重稀土分馏明显(LREE/HREE为5.1-10.5),明显亏损Eu(δEu=0.19-0.26),稀土元素配分模式呈右倾海鸥型,Sr、Ba、Ti等元素的强烈亏损,与典型的A型花岗岩一致。岩石地球化学研究表明,蕴都卡拉一带A型花岗岩可能是下地壳先存的俯冲洋壳或富Nb玄武岩部分熔融的产物,形成于晚石炭世后造山的拉张构造环境,是准噶尔东北缘造山作用结束的标志。

Eastern Junggar of Xinjiang Uygur Autonomous Region is an important area of the Central Asian Orogenic belt, where the Siberia and Kazakhstan plates collided. Late Carboniferous granites are widespread in this area, among which alkali granites in Yundukala are investigated and studied in this paper. Yundukala alkali granites are characterized by high SiO 2 (72.40%-74.17%), high alkaline (Na 2O+K 2O=8.41%-10.40%), low TiO 2 (0.20%-0.40%) and low CaO (0.38%-0.70%) contents. They are significantly enriched in high field strength elements like Zr, Th, and Y, and exhibit highly differentiated REE patterns, showing right dipping lines with strong negative Eu anomaly, whilst their obvious depletion of Sr, Ba and Ti might have resulted from fractional crystallization of feldspar and Ti-oxides. These geochemical characteristics indicate they are typical A-type granites and may be the product of partial melting of the predominantly subducted oceanic crust or Nb-rich basalt in the lower crust. The formation of these A-type granites infers that collisional orogenesis ended during late Carboniferous.